1. Field of the Invention
The invention relates to a device that simultaneously produces several beams of light corresponding to different spectral domains.
The invention can be applied particularly (but not exclusively) to the illumination of screens for the spatial modulation of light in color image projectors.
2. Description of the Prior Art
In video projectors of images, whether colored or not, the image to be projected is formed on the surface of the light spatial modulator screen. This screen is generally formed by a liquid crystal matrix cell. The light given by a light source is spatially modulated by the matrix, and the projected image results from this modulation.
One of the problems of these projectors is that their luminous efficiency is low, of the order of a few per cent, and this problem is even more marked with color projectors. For, in many color projectors, the polychromatic image is obtained by the superimposition of three monochromatic images. In this type of color projector, it is standard practice to use a single source producing a beam of white light or primary beam and to separate this primary beam into three monochromatic beams, each corresponding to one of the primary colors. Each monochromatic beam is modulated by a spatial modulator, and the three modulated monochromatic beams, each corresponding to one of the primary colors, are then superimposed to form the image.
The low luminous efficiency of the image projectors is related to different causes, one of which lies in the light losses due to the differences between the shape of the light modulator screen to be illuminated and the shape of the section of the beam coming from the light source.
Thus, for example, in the context of the development of high-definition, wide-screen television sets, the image is designed with a 16/9 format (the ratio of the length to the width). An image-projecting system meeting this definition uses a spatial modulator screen having this 16/9 format. In this case, assuming, in the most common example, that the light beam coming from the source has a circular section, the fraction of light energy used to illuminate the rectangle constituted by the spatial modulator screen corresponds only to the SR/SF ratio of the area SR of this rectangle to the area SF of the section of the beam, giving 0.54 at best, i.e. when SR is as great as possible, at the format 16/9 while, at the same time, being contained in SF.